Shower tray installation and Tile backer board under shower tray or butted upto it?

22 Nov 2016
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United Kingdom

The floor is 18mm WBP Ply fixed to the floor joists.

Is it best to fit hardie backer board over the whole floor, and then fit the shower tray onto the backer board, or fit the tray onto the ply floor then butt the backer board upto the edge of the tray?

Also, the tray instructions say to fit the tray onto minimum 18mm Ply using a sand and cement mix.
Ive only ever fitted trays onto a sand and cement mix but I see quite a few posts of people using tile adhesive and some even using silicone/nonails.
Whats the best way to fit the tray to either the ply or the backer board? Stick with the sand/cement method or go with tile adhesive?
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I'd drop the tray straight on to the ply if it's nice and flat/level and hardie up to it's edge. Use an aquastrap or similar to attach shower to all wall edges. Use a wetgrab or similar to bed tray onto ply, good quality silicone seal all the way around the tray and all leading edges against the walls and floors. Then fit hardie and seal again. Sand and cement or cement based adhesives IMO are no longer needed give the newer types of grab adhesives/mastics that are now available.
Thanks for replies.
Ive read through the ditra info.....
So would you use ditra matt instead of the hardieboard and go straight onto the 18mm ply, or aswell as the hardie and put it ontop of that?

Ply floor is level and rock solid.

Ive Never used or seen matting straight on 18mm ply before?
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Straight on the ply... unless you want a stepladder to get into the bathroom!
so you would use matting instead of the hardieboard?
i know backerboard adds some strength to the floor which is usually the idea, however the matting doesnt. it only helps for horizontal movement etc.
i was originally going to use backer board and no matting, however should i just go with matting instead then?

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